The present invention generally relates to intravascular stents and more particularly pertains to improvements thereto that provide for enhanced longitudinal flexibility, increased longitudinal stability upon radial expansion and high strength.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency. These devices are typically intraluminally implanted by use of a catheter which is inserted at an easily accessible location and then advanced to the deployment site. The stent is initially in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed which, depending upon is configuration, is achieved either automatically or actively by for example, the inflation of a balloon about which the stent is carried on the catheter.
As stents are normally employed to hold open an otherwise blocked, constricted or occluded lumen, a stent must exhibit sufficient radial or hoop strength in its expanded state to effectively counter the anticipated forces. Not only is it advantageous to distribute such loads over as much of the stent as possible but it also is most beneficial to distribute the load over as much lumen wall as possible. As a consequence, it is desirable to maximize the coverage area of the stent in its expanded state. It is, however,. simultaneously necessary for the stent to be as small and compact as possible in its collapsed state in order to facilitate its advancement through the lumen. As a result, it is most advantageous for a stent to have as large an expansion ratio as possible.
An additional consideration is the longitudinal flexibility of the device. Such characteristic is important not only in maneuvering the stent into position, which may require the traversal of substantial convolutions of the vasculature, but also to better conform to any curvature of the vasculature at the deployment site. At the same time it is, however, necessary for the stent to nonetheless exhibit sufficient radial strength to provide the necessary support for the lumen walls upon deployment.
Another problem inherent in many prior art stent configurations is the longitudinal contraction that such structures typically undergo as they are radially expanded. This not only reduces the effective length of the stent in its deployed state but may cause abrasion trauma to be inflicted on the vessel walls during expansion.
A number of very different approaches have been previously devised in an effort to address these various requirements. A popular approach calls for the stent to be constructed wholly of wire. The wire is bent, woven and/or coiled to define a generally cylindrical structure in a configuration that has the ability to undergo radial expansion. The use of wire has a number of disadvantages associated therewith including for example, its substantially constant cross-section which may cause greater or lesser than an ideal amount of material to be concentrated at certain locations along the stent. Additionally, wire has limitations with respect to the shapes it can be formed into thus limiting the expansion ratio, coverage area, flexibility and strength that can ultimately be attained therewith.
As an alternative to wire-based structures, stents have been constructed from tube stock. By selectively removing material from such tubular starting material, a desired degree of flexibility and expandability can be imparted to the structure. Chemical etching techniques as well as laser-cutting processes are utilized to remove material from the tube. Laser cutting provides for a high degree of precision and accuracy with which very well defined patterns of material can be removed from the tube to conversely leave very precisely and accurately defined patterns of material in tact. The performance of such stent is very much a function of the pattern of material which remains. The selection of a particular pattern has a profound effect on the coverage area, expansion ratio and strength of the resulting stent as well as its longitudinal flexibility and longitudinal dimensional stability during expansion.
While the tube-based stents offer many advantages over the wire-based designs, it is nonetheless desirable to improve upon such designs in an effort to further enhance longitudinal flexibility and longitudinal dimensional stability during radial expansion without sacrificing radial hoop strength.
The present invention provides for an improved tube-based stent having enhanced longitudinal flexibility and longitudinal dimensional stability during radial expansion while exhibiting adequate hoop strength. The improvements arise with the selection of a precisely defined pattern of voids that are cut or etched into tube stock. The pattern of material that remains to define the stent comprises a series of generally parallel serpentine elements wherein such elements are interconnected to one another by advantageously shaped and positioned bridging members. More particularly, each serpentine element extends circumferentially about the stent such that successive apexes of each element alternatively extend distally and proximally along the stent""s surface. The serpentine elements are successively spaced along the length of the stent and are oriented such that the pattern of apexes of each element is 180xc2x0 out of phase relative to the pattern of apexes of each directly adjacent element. Each of the serpentine elements are joined to an adjacent element by at least one bridging member. The number, length and flexibility of such members determine the longitudinal flexibility of the resulting device in its collapsed as well as deployed state. The attachment of the bridging members to juncture points along the linking segments extending between the apexes causes the bridging members to distort during deployment. By matching the dimensional change imparted to the length of the stent caused by the expansion of each serpentine element to the dimensional change along the same direction caused by the rotation and distortion of the bridging element, the overall length of the stent is held constant during deployment. Additionally, the spacing of the juncture points away from the apexes distributes the stress that would otherwise be concentrated at such locations during expansion of the device.
These and other features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention.